1. Field of the Invention
This invention is concerned with an optical amplifier for use with a wavelength division multiplex optical communications system.
2. Description of the Related Art
In association with an optical communications system, it is well known that a long distance transmission of optical signals is achieved via use of optical amplifiers disposed along an optical transmission line at intervals of a certain distance. For example, a transmission line crossing the Pacific Ocean is provided with several tens to several hundreds of optical amplifiers along the way. Among various types of optical amplifiers developed and used for such a purpose, the most popular is the type that makes use of rare-earth element doped optical fibers (hereinafter called xe2x80x9crare-earth doped fiber optical amplifiersxe2x80x9d).
Expansion in the transmission capacity of communications systems is absolutely necessary to meet the rapid increase in the volume of information that is exchanged through communication networks such as the Internet. The Wavelength Division Multiplexing (WDM) transmission is one of the technologies being developed to realize such a large capacity of transmission systems, and has already been employed on a commercial scale.
In a WDM transmission system, a plurality of optical signals with different wavelengths are multiplexed and transmitted through a single optical fiber. Thus, a certain wavelength band is needed for the plurality of optical signals. In practice, for example, bands called a C-band (1530-1560 nm) and an L-band (1560-1620 nm) are known. Therefore, an optical amplifier for WDM system is required to collectively amplify all optical signals within the C-band and/or L-band. Among rare-earth doped optical fiber amplifiers, erbium doped optical fiber amplifiers are the most common ones used for amplifying optical signals within the C-band or L-band. For this reason, our discussion on optical amplifiers below is concerned with erbium doped fiber optical amplifiers.
FIG. 1 shows the performance of a conventional erbium doped fiber optical amplifier, in particular, the performance of it when multi-wavelength light is amplified. The length of an erbium doped fiber is shown on the horizontal axis, and the optical powers at points along the erbium doped fiber are shown on the vertical axis for the various wavelengths. In addition, optical power of each of the input signals in the multi-wavelength light is xe2x88x9220 dBm.
When multi-wavelength light is amplified by an erbium doped fiber optical amplifier, a deviation of the gain with respect to wavelength depends on the length of the erbium doped fiber. When the erbium doped fiber is short, the shorter the wavelength of an input light, the larger the gain becomes. In the example shown in FIG. 1, 88 optical signals with the optical power of xe2x88x9220 dBm/ch are input, and pump light with 980 nm is provided in a same direction as the signals. The optical power deviation exceeds 10 dB when the erbium doped fiber is about 10 meters. In contrast, when the erbium doped fiber is longer, the gain deviation becomes smaller, as the gains for shorter wavelength light tend to fall as the fiber becomes longer. In the case of the example shown in FIG. 1, when the erbium doped fiber length is 40 meters, the optical power deviation is reduced to around 5 dB.
FIG. 2 shows the average gain and the gain tilt of an erbium doped fiber optical amplifier. The average gain here means the average of gains measured respectively for a plurality of optical signals being amplified by an erbium doped fiber optical amplifier. The gain tilt means the difference between gains for the shortest wavelength (e.g., 1570 nm) and the longest wavelength (e.g., 1607 nm) among a plurality of optical signals that are amplified by an erbium doped fiber optical amplifier.
The average gain obtained by an erbium doped fiber optical amplifier, as shown in FIG. 2, stays fairly unchanged for erbium doped fiber lengths of 10 through 40 meters, while the gain tilt becomes the smallest when the erbium doped fiber length is about 40 meters. Here, the average gain should be large in order to obtain large output power of an erbium doped fiber amplifier. The gain tilt should be small in order to evenly amplify a plurality of optical signals with different wavelength. Based on the consideration of these factors, in an erbium doped fiber optical amplifier for amplifying L-band, it is preferable that length of the erbium doped fiber is 40 meters, under the conditions assumed here.
FIG. 3 shows the gain of an erbium doped fiber optical amplifier with respect to the inverted population. It is desirable that gain is constant (or flat) in a wavelength range corresponding to the L-band in order to evenly amplify all the optical signals within the L-band. Therefore, it is understood that the inverted population of an erbium doped fiber is best to be set at about xe2x80x9c0.4xe2x80x9d for amplifying L-band optical signals. However, the gain for a unit fiber length is small when the inverted population is about xe2x80x9c0.4xe2x80x9d. Thus, the length of an erbium doped fiber needs to be relatively long in order to obtain a large gain with the erbium doped fiber.
In this way it is imperative that the length of an erbium doped fiber becomes long to some extent if an erbium doped fiber optical amplifier is used for amplifying optical signals of L-band.
In FIG. 1, it is shown that the optical power of an optical signal with shorter wavelength becomes the maximum when the length of an erbium doped fiber is about 10 meters, and the optical power gradually declines as the length increases. This phenomenon is explained as being caused by the energy of optical signals with the shorter wavelength (for example, optical signals of 1570-1580 nm) being absorbed by optical signals with the longer wavelength (for example, optical signals of 1600-1607 nm). In other words, the shorter wavelength optical signals are serving as pump light for the longer wavelength optical signals.
In a WDM transmission system, mutually different wavelengths are allocated to a plurality of transmission channels. Therefore, when a new transmission channel is added, new optical signal with a corresponding wavelength is added to a transmission line. On the other hand, when an existing transmission channel is removed, transmission of the corresponding wavelength is stopped.
As described above, the signal light with shorter wavelength works as pump light for the signal light with longer wavelength in the L-band. Therefore, if a transmission channel corresponding to a signal light with shorter wavelength is stopped, a phenomenon as if optical power of pump light became smaller for signal lights with longer wavelengths in the erbium doped fiber optical amplifier.
FIG. 4 shows an interaction between optical signals of mutually different wavelengths. Here, it is assumed that only signal lights with 1570 nm and 1584 nm are transmitted. Profile-a shows the optical power of the 1570 nm signal light, when transmitting both 1570 nm and 1584 nm signal lights. Profile-b shows the optical power of the 1584 nm signal light, when transmitting both 1570 nm and 1584 nm signal lights. Profile-c shows the optical power of the 1584 nm signal light, when transmitting only the 1584 nm signal light.
As is clearly understood from these profiles, the optical power of 1584 nm signal light varies markedly depending on the presence or absence of 1570 nm signal light. In particular, in a case that erbium doped fiber is 40 meters, the optical power of 1584 nm signal light is reduced by more than 5 dB, when the 1570 nm signal light is stopped.
When amplifying optical signals within the L-band using an erbium doped fiber optical amplifier, terminating an optical signal with the shorter wavelength results in reduction in the optical power of optical signal with longer wavelength. Then, if output power of the optical signal is small, the optical signal may not be detected by a receiver (receiving station or optical receiver in the receiving station).
As a solution to this problem, a configuration with a function for adjusting the optical powers of each wavelength (such as an automatic gain controller: AGC) is considered. According to this configuration, the power reduction in an optical signal with the longer wavelength that results from termination of an optical signal with the shorter wavelength is automatically compensated. However, with this configuration, it takes some time until the optical power of an optical signal is adjusted to an optimum level. In addition, a response speed of a control circuit is restricted in order not to compete with the response speed of the doped rare-earth ion (here, that of erbium). Therefore, it is difficult to avoid occurrences of temporary shutdown of optical signal with the longer wavelength even the adjusting function is introduced.
The objective of this invention is to reduce output fluctuations of optical amplifiers used in a WDM optical communications system.
An optical amplifier of the present invention used in a WDM optical communications system, comprises: an optical fiber amplifying multi-wavelength light including a plurality of optical signals, a pump light source supplying pump light to the optical fiber, a dummy light source supplying dummy light to the optical fiber, an input monitor detecting a optical power of input light to the optical fiber, an output monitor detecting optical powers of the plurality of optical signals which have been amplified in the optical fiber, and a controller controlling the optical power of the pump light generated by the pump light source according to optical powers detected by the input monitor and the output monitor.
The optical amplifier collectively amplifies the multi-wavelength light including the plurality of optical signals. When this multi-wavelength light is amplified, some of the optical signals in the multi-wavelength light serve as pump light for some other optical signals. The dummy light is supplied to serve as pump light in the same way as some of the optical signals. Therefore, even some of the optical signals are stopped, the changes in the output optical powers of other optical signals are small, as they are pumped by the dummy light.
An optical amplifier of another feature of the present invention comprises: an optical fiber amplifying multi-wavelength light including a plurality of optical signals, a pump light source supplying pump light to the optical fiber, a dummy light source supplying dummy light to the optical fiber, and a controller controlling the dummy light source according to a number or an allocation of the optical signals in the multi-wavelength light.
In this optical amplifier, the optical power of the pump light should be large, if a number of optical signals in the multi-wavelength light is large. Therefore, if the number of optical signals in the multi-wavelength light is sufficiently large, when a particular optical signal is stopped, the changes in the output optical powers of other optical signals are small even without the dummy light.
Under situations in which the plurality of optical signals are allocated within a particular wavelength region, no optical signal serves as pump light for other optical signals. Therefore, when some optical signals are stopped, the changes in the output optical powers of the other optical signals are small even without the dummy light.
As a results, if the dummy light source is controlled according to the number or the allocation of the optical signals, generation of unnecessary dummy light is avoided.